Fix ODR violation that can break WTF::String pretty printing in tests.

Source/wtf/text/WTFString.h

 /*
  * (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012, 2013 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 13 matching lines @@
 
 // This file would be called String.h, but that conflicts with <string.h>
 // on systems without case-sensitive file systems.
 
 #include "wtf/HashTableDeletedValueType.h"
 #include "wtf/WTFExport.h"
 #include "wtf/text/ASCIIFastPath.h"
 #include "wtf/text/StringImpl.h"
 #include "wtf/text/StringView.h"
 #include <algorithm>
+#include <iosfwd>
 
 #ifdef __OBJC__
 #include <objc/objc.h>
 #endif
 
 namespace WTF {
 
 class CString;
 struct StringHash;
 
@@ skipping 582 matching lines @@
 template<typename T> struct DefaultHash;
 template<> struct DefaultHash<String> {
     typedef StringHash Hash;
 };
 
 // Shared global empty string.
 WTF_EXPORT const String& emptyString();
 WTF_EXPORT const String& emptyString16Bit();
 WTF_EXPORT extern const String& xmlnsWithColon;
 
+// Pretty printer for gtest. Declared here to avoid ODR violations.
+WTF_EXPORT std::ostream& operator<<(std::ostream&, const String&);

[dcheng, 2015/07/27 19:59:04]

It feels a little sketchy to declare an exported function in one component and
define it in another, even if it does compile.

An alternative I explored was using PrintTo(), but unfortunately, that means
things like:

EXPECT_EQ(a, b) << "more info about b: " << b->fieldThatReturnsString();

need to be rewritten to:
EXPECT_EQ(a, b) << "more info about b: " <<
::testing::PrintToString(b->fieldThatReturnsString());

Ultimately, this felt like the least bad solution.

+
 } // namespace WTF
 
 WTF_ALLOW_MOVE_AND_INIT_WITH_MEM_FUNCTIONS(String);
 
 using WTF::CString;
 using WTF::KeepTrailingZeros;
 using WTF::StrictUTF8Conversion;
 using WTF::StrictUTF8ConversionReplacingUnpairedSurrogatesWithFFFD;
 using WTF::String;
 using WTF::emptyString;
@@ skipping 12 matching lines @@
 using WTF::charactersToFloat;
 using WTF::equal;
 using WTF::equalIgnoringCase;
 using WTF::find;
 using WTF::isAllSpecialCharacters;
 using WTF::isSpaceOrNewline;
 using WTF::reverseFind;
 
 #include "wtf/text/AtomicString.h"
 #endif // WTFString_h